The present invention relates to a method of influencing the cis/trans isomer ratio of 3-aminomethyl-3,5,5-trimethylcyclohexylamine (isophoronediamine; IPDA) in its preparation from 3-cyano-3,5,5-trimethylcyclohexanone (isophoronenitrile; IPN). The preparation of isophoronediamine is based on the aminating hydrogenation reaction of isophoronenitrile with ammonia and hydrogen in the presence of a hydrogenation catalyst.
Isophoronediamine is used as a starting material for the preparation of isophorone diisocyanate which is utilized as an isocyanate component for polyurethane systems, as an amine component for polyamides, and as a hardener for epoxy resins. Isophoronediamine is conventionally prepared from isophoronenitrile, the carbonyl group being converted to an amino group and the nitrile group to an aminomethyl group in the presence of ammonia, hydrogen and conventional hydrogenation catalysts. The starting material, isophoronenitrile, can be obtained in a known manner by the addition of hydrogen cyanide onto isophorone (see U.S. Pat. No. 5,091,554 which is incorporated by reference in its entirety; DE-OS 39 42 371).
Isophoronediamine can exist in different isomeric forms. According to "Die Angewandte Makromolekulare Chemie", 153 (1987) 1-13 (no. 2502), the isomer present to the extent of about 75% in commercially available isophoronediamine was found to have a chair conformation with a cis arrangement of the equatorial amino group on the C.sup.1 atom and the equatorial aminomethyl group on the C.sup.3 atom; the isomer present to the extent of about 25% is the trans isomer with an equatorial amino group and axial methylamino group.
The cis and trans isomers of isophoronediamine, and the cis and trans isomers of isophorone diisocyanate obtainable therefrom, have different reactivities which can be of technical importance for the proposed utilities for these components. DE-OS 42 11 454 (CA 2,093,374) teaches that by using a mixture of isophoronediamine isomers consisting of more than 40% of the trans isomer and less than 60% of the cis isomer as a reaction component in polyaddition resins, such as epoxy resins in particular, the pot life of the resins is lengthened and the maximum hardening temperature thereof is lowered. Conversely, to achieve the fastest possible reaction rate, the preferred mixtures of isophoronediamine isomers are those containing the highest possible proportion of the cis isomer.
The preparation of isophoronediamine from isophoronenitrile is described in many references which say nothing about the composition of the isomer mixture; whereas commercially available products have a cis/trans isomer ratio of about 75 to 25, DE-OS 42 11 454 has disclosed mixtures of IPDA isomers which contain between 50 and 70% of the trans isomer.
As regards known processes for the preparation of isophoronediamine by the aminating hydrogenation reaction of isophoronenitrile, reference is made by way of example to U.S. Pat. No. 3,352,913 (which is incorporated by reference in its entirety), EP-B 0 042 119 (U.S. Pat. No. 4,429,157), EP-A 0 449 089 (CA 2,039,328), EP-A 0 394 967 and JP-A 4-300852. None of these references gives any information on the isomer ratio.
According to the one step process described in U.S. Pat. No. 3,352,913, isophoronenitrile is subjected to aminating hydrogenation with ammonia at 50.degree. to 150.degree. C. and at a pressure of at least 5 MPa (e.g., 12 to 15 MPa) in the presence of cobalt-, nickel-, iron- or noble metal-containing catalysts known per se. The hydrogenation can be carried out in the presence or absence of organic solvents, methanol being preferred. JP-A 4-300852 also discloses a one step process.
DE-OS 30 11 656 teaches a two-step reaction in which isophoronenitrile is converted to 3-cyano-3,5,5-trimethyliminocyclohexane in the first step and the latter is hydrogenated to IPDA in the second step. EP-B 0 042 119 also relates to a two-step process for the preparation of IPDA from IPN using a special imine formation reactor. In EP 0 449 089 (CA 2,039,328) there is also described a two-step process wherein a special catalyst is used in the first step of forming an imine.
Another two-step process is the one described in EP-A 0 394 967 for the amination of carbonylnitriles and iminonitriles, which also includes the preparation of IPDA from IPN. The starting material is initially converted to the aminonitrile under conditions of reductive amination (i.e., in the presence of hydrogen, ammonia and a hydrogenation catalyst, and conventionally in the presence of an organic solvent) at moderate temperatures; the nitrile group is then converted to an aminomethyl group at elevated temperature in the presence of a hydrogenation catalyst which is effective in the hydrogenation of nitrile groups. As is apparent from the numerous Examples in EP-A 0 394 967, it was considered necessary to operate a strict temperature program; the temperature of the first step being increased stepwise in 20.degree. C. intervals to the temperature of the second step. The time-consuming temperature program leads to a lowering of the space-time yield and hence a reduction in the economy of the process. Special promoters were additionally used in order to achieve an adequate product quality.